iPad Mini 4, September 2015: 16GB, 64GB, 128GB storage capacities.
iPad Mini 4, September 2016: 32GB and 128GB storage capacities.
Battery replacement is not easy, so of you buy used check out the battery health with coconutBattery if you can.
A few weeks ago I scored a killer deal on a 2015 iPad Mini 4 with 90% battery health and only 47 load cycles. It's nice but I'll sell it and wait for the iPad Mini Pro in March. If the display were better I'd keep it but the contrast sucks compared to my Nexus 7. Too much eye strain compared to my Nexus 7. It's better than the N7 in every other way but I read a lot so it's worth the wait for the Mini Pro.